Diurnal and Daily Changes in the Levels of Sesquiterpene Lactone and Other Components in Lettuce Post-Harvest

Lettuce, which contains sesquiterpene lactones that have been associated with anti-inflammatory and sedative properties, also appears to harbor bitter ingredients such as lactucopicrin, often found abundantly in the emulsion released from the cut core. Previous reports suggest that lettuce may gradually increase in bitterness post-harvest, possibly reflecting alterations in the composition of its components during shelf life. Therefore, analyzing changes in the concentrations of these components could contribute to the development of methods for evaluating lettuce freshness. In this study, we examined variations in sugar contents and hydrophilic oxygen radical absorbance capacity values in lettuce leaves, refined an analytical approach for sesquiterpene lactones in the lettuce core, and explored how their levels may differ depending on harvest timing within the same day and across the storage period. High-resolution LC-MS analysis was employed to estimate the levels of key components such as cichorioside B, 11β,13-dihydrolactucin, lactucin, and lactucopicrin. While the emulsion is generally considered to contain substantial amounts of lactucopicrins, relatively little information is available about the components present in the lettuce core. Our current findings indicate that cichorioside B may be a predominant bitter component in the core. Collectively, these results may provide a basis for developing approaches to assess lettuce freshness and monitor compositional changes during storage.